In a digital transmission system, there is a need to monitor the quality of the signal transmitted. The monitor should indicate whether or not the quality of the modulated signal as it leaves the transmitter is sufficient to be processed by a distant receiver in an error free manner. If the quality factor is not high enough, then corrective action can be taken or stand-by equipment can be switched on line to protect the data.
One way of providing quality monitoring is by using a full receiver at the transmit terminal for fully demodulating the transmitted signal back to baseband to recover the data. The use of a receiver is cumbersome and expensive. For example, in the case of quadrature phase shift keyed modulation (QPSK), the receiver has a pair of phase detectors, one for each quadrature demodulation axis, and the appropriate data processing and coordinating circuitry therebetween, in addition to carrier recovery circuitry and clock recovery circuitry. Furthermore, because of the ambiguity in establishing the 0.degree. reference phase of the recovered carrier, the signal may be differentially encoded for sending the differences in phase rather than absolute phase position to carry the data, and hence decoding circuitry is needed. Still further, the reliability of the monitoring receiver is the same order or less than the transmitter and hence detracts from the overall system reliability.
Another known monitoring technique involves the use of activity monitoring equipment on the digital baseband. Activity monitors are simple and reliable; however, they do not monitor the quality of the modulated signal, but rather only whether or not there is any data activity on the baseband (i.e., transitions from logic 1 to logic 0). These activity monitors do not provide any indication of whether or not the carrier signal has been properly modulated.